The present invention relates to a bearing, and in particular, to a hybrid foil bearing.
Air cycle machines are used in environmental control systems in aircraft to condition air for delivery to an aircraft cabin. Conditioned air is air at a temperature, pressure, and humidity desirable for aircraft passenger comfort and safety. At or near ground level, the ambient air temperature and/or humidity is often sufficiently high that the air must be cooled as part of the conditioning process before being delivered to the aircraft cabin. At flight altitude, ambient air is often far cooler than desired, but at such a low pressure that it must be compressed to an acceptable pressure as part of the conditioning process. Compressing ambient air at flight altitude heats the resulting pressured air sufficiently that it must be cooled, even if the ambient air temperature is very low. Thus, under most conditions, heat must be removed from air by the air cycle machine before the air is delivered to the aircraft cabin. A cabin air compressor can be used to compress air for use in an environmental control system. The cabin air compressor includes a motor to drive a compressor section that in turn compresses air flowing through the cabin air compressor.
Both air cycle machines and cabin air compressors have a shaft extending down a central axis that rotates. Bearings are positioned outward from the shaft to reduce friction between the rotating shaft and stationary components. Historically, ball bearings were used in air cycle machines and cabin air compressors. Ball bearings face limitations in that they wear out quickly and thus need to be replaced often. Further, ball bearings require oil for operation and the smell of the oil can seep into the air flowing through the air cycle machine and/or cabin air compressor before the air is delivered to the aircraft cabin.
To overcome the limitations of ball bearings, air bearings were later developed for use in air cycle machines and cabin air compressors. Air bearings create an air gap between a rotating part and the bearing components so that the air gap acts as the bearing. Examples of air bearings that can be used are bump foil bearings and metal mesh bearings. Bump foil bearings include a bump foil positioned between a top foil and a bearing sleeve. Metal mesh bearings include a metal mesh positioned between a top foil and a bearing sleeve. With both bump foil bearings and metal mesh bearings the top foil is positioned around the shaft. As air flows along the shaft, the top foil is pushed outward from the shaft to create an air gap between the rotating shaft and the top foil. Bump foil bearings have a high stiffness and can support high loads but have low dampening characteristics. The low dampening characteristics can lead to a phenomenon known as sub-synchronous whirl, which is the problem of uncontrolled vibration of the shaft. Metal mesh bearings have high dampening characteristics, but sag over time causing the shaft to become off centered.